Studies support a prominent role of stress in irritable bowel syndrome (IBS). The principal branches of the general stress response are the hypothalamic-pituitary-adrenal (HPA)axis and the sympathetic nervous system (SNS). The overall hypothesis of this Project is that females and males differ in their HPA axis responsiveness and SNS activation, particularly under stress conditions, and that these differences will help explain the greater vulnerability of females to develop chronic stress-related conditions such as IBS. Furthermore, these sex-related differences in the HPA and SNS are reflected in differences in colonic mucosal immune modulation which in turn relates to the higher prevalence of colonic visceral hypersensitivity in female IBS patients. We will study four subject groups: female IBS patients, male IBS patients, female controls and male controls. In Aim 1, we will test the hypothesis that under basal conditions, males have greater HPA (ACTH, cortisol) and general SNS (cardiosympathetic and sympathoadrenal) activity but females may have greater SNS activity to the pelvic organs. Specifically, we will compare HPA responsiveness in males and females using HPA axis challenge tests, biomathematical modeling analysis to characterize the diurnal rhythm of ACTH and cortisol, and estimate chronically enhanced SNS output to the colon by measuring tissue NE levels and alpha 2-receptor density in colonic biopsies. In Aim 2, we will test the hypothesis that under stress conditions, males show greater general measures of plasma HPA (ACTH, cortisol) and SNS activity (cardio-sympathetic, catecholamines and skin conductance) responses to a visceral stressor (flexible sigmoidoscopy) and a psychological stressor (Trier social stress test) compared to females. In Aim 3, we :will test the hypothesis that symptomatic IBS patients have: altered plasma and mucosal cytokine production which differ in females and males and are related to sex-related differences in visceral pain perception. We will measure plasma and colonic mucosal cytokine markers, and perceptual responses to rectal balloon distension to determine if visceral sensitivity is associated with mucosal and plasma immune markers. The combination of experimental approaches should improve our understanding of the sex-related differences in stress response mechanisms underlying functional pain syndromes.